It is well known that peroxide compounds, such as hydrogen peroxide, can occur as unwanted constituents in a number of process or wastewater streams. Hydrogen peroxide, for example, is toxic to certain sensitive ecosystem components such as Daphnia at concentrations of a few parts per-million. The presence of peroxide in certain process streams may cause unwanted degradation of key system components or materials of construction. Thus, the removal of such peroxide compounds may not only be desirable, but also necessary.
Past practices for the removal of peroxides have relied either on direct chemical reduction or the use of materials, such as activated carbon, which are capable of catalyzing the direct autoxidation-reduction of the peroxide moiety. The practice of using direct chemical reduction is often expensive and inconvenient, and usually requires continuous chemical addition and continuous monitoring of chemical dose rates as a function of peroxide concentration. The use of activated carbon, however, is inherently more convenient since it requires no chemical additions. For this reason it is preferred when it can be made economically viable.
It has been known that certain activated carbons have the ability to catalyze peroxide decomposition and removal, the extent to which such carbons can effect peroxide removal has not been sufficient to allow their widespread use per se in such applications. In fact, some carbons are so inactive towards peroxides that they can be used to purify the peroxide streams without significant peroxide decomposition. To obtain satisfactory commercial performance in peroxide removal applications it has usually been necessary to impregnate activated carbons with metals such as platinum or palladium. Such metals are highly active for peroxide decomposition. Other carbon post-treatments, such as exposure to nitrogen-containing compounds at high temperatures, have in some cases also resulted in improvements in carbon performance. However, each of these methods has certain disadvantages which have limited their widespread commercial acceptability.
Where activated carbons have been impregnated to improve their peroxide removal capabilities, several inherent disadvantages limit their overall utility. These disadvantages include the high costs of the impregnants, low carbon ignition temperatures, impregnant toxicity and attendant disposal limitations, and the tendency of many impregnants to leach into, dissolve into, or otherwise contaminate liquid effluent process streams.
Where activated carbons have been post-treated at high temperatures with nitrogen-containing compounds to improve activity, the processes required to produce enhanced activity have been inherently expensive and hazardous, yielding products of variable quality and marginal economic utility. Typically, such processes are expensive because they employ a nitrogen-rich synthetic compound or a finished high-temperature char, such as an activated carbon, as the primary feedstock, or require large quantities of reagents, large carbon losses, and significant departures from standard activated carbon production practices to effect significant gains in the peroxide reactivity of the final product. Additionally, such processes are hazardous because they usually employ hazardous reagents, such as ammonia or nitric acid, and generate significant amounts of toxic byproducts, such as cyanide or nitrogen oxides, during processing.
Accordingly, it is the object of the present invention to provide a process for peroxide decomposition and removal which is at once economical, convenient, effective, and environmentally benign. It is further the object of the present invention to employ a carbon for this process which is made directly from an inexpensive and abundant nitrogen-poor starting material such as a bituminous coal or a bituminous coal-like material, and to limit the use of agents responsible for imparting peroxide reactivity to the starting material by performing the essential treatment steps during the low-temperature transition of the starting material into the final product. It is yet a further object of the invention to provide carbon treatment steps which include the use of low-temperature carbonization and oxidation of the starting material, by inexpensive, abundant, and relatively non-toxic oxidants, and exposure of the oxidized, low-temperature char to small amounts of inexpensive, abundant and relatively non-toxic nitrogen-containing compounds during, rather than after, the initial calcination and condensation of the carbon structure. Such treatments are highly compatible with current processes for manufacturing activated carbons, and can be carried out with minimal departures from conventional practice.